In situ gelation of polymers during waterflooding

ABSTRACT

A method for plugging waterflood zones through the in situ production of gelatinous materials derived from injecting a solution containing a water-soluble bivalent metal salt and a water-soluble polymer of high molecular weight having groups chemically reactive with trivalent metal ions into a waterflood zone followed by the injection of air which converts bivalent metal ions to trivalent metal ions thereby resulting in the aforementioned gelatinous plugging material.

United States Patent Lanning et al.

[451 Apr. 25, 1972 [54] IN SITU GELATION OF POLYMERS DURINGWATERFLOODING [72] Inventors: William C. Lanning; James W. Gall, both ofBartlesville, Okla.

[73] Assignee: Phillips Petroleum Company [22] Filed: Sept. 1, 1970 211Appl. No.: 68,769

[52] U.S. Cl ..166/270, 166/295, 166/294 [51] Int. Cl. ..E2lb 33/138,1321b 43/20 [58] Field of Search ..166/269, 268, 270, 273, 274,

[56] References Cited UNITED STATES PATENTS 3,370,649 2/1968 Wogelmuth..166/274 3,378,070 4/1968 Wesslereta] ..l66/294X 3,396,790 8/1968 Eaton[66/270 3,400,761 9/ 1968 Latimer, Jr. et a1 ..166/294 X 3,406,75410/1968 Gogarty ..l66/273 3,502,149 3/1970 Pence, Jr. ..l66/2953,580,337 5/1971 Gogarty ..l66/274 Primary Examiner-Stephen J. NovosadAttorneyYoung and Quigg [57] ABSTRACT 7 Claims, No Drawings IN SITUGELATION F POLYMERS DURING WATERFLOODING This invention relates to thetreatment of high permeability strata through which flooding water tendsto bypass crude oil in waterflooding operations. In particular, thisinvention relates to a method for selectively plugging the morepermeable waterflow channels of an earth formation. The method of thisinvention is useful in promoting better control of fluid injectionpatterns as are desired and the secondary recovery of petroleum bydisplacement with floodwater.

Since only a portion of the oil contained in a petroleum reservoir canbe recovered by primary methods, it has become conventional practice toemploy various secondary and tertiary recovery techniques in order tofully exploit the quantities of oil contained in said reservoirs. Of thevarious secondary and tertiary recovery methods currently available, oneof the most widely practiced techniques is the displacement of the oilfrom the formation by a displacement fluid injected for that purpose,commonly called waterflooding. While waterflooding has been ratherwidely adopted in recent years, it is not without considerable operatingproblems and economical limitations. Some of the above problems anddifficulties are caused by the aqueous displacing medium channelingand/or fingering through the oil-bearing structure to the producingwell, thus bypassing large zones of the oil-bearing strata. While a moreuniform flood front with reduced fingering can be obtained with adisplacement fluid rendered viscous by the addition of variouswater-soluble materials, viscous waterflooding alone is not fullyeffective in all formations.

More uniform flood fronts can be obtained in formations of nonuniformpermeability by control or adjustment of the permeability of the morepermeable strata of the formation. A number of methods of reducing thepermeability of these permeable strata have been proposed, including theinjection of plugging materials into the strata which at least partiallyplug the permeable zone so as to achieve more uniform permeability.Water-soluble polymers are thought of as functioning in one or more ofthe following ways when utilized in an oil recovery waterfloodoperation: (1) simple increase in water viscosity, (2) reducedpermeability due to polymer absorption on the rock surface, and (3)plugging of pores by quite large polymer molecules or gels of polymermolecules.

The method of our invention provides means of improving performance ofwaterflood operations by pore plugging. This approach has the advantagethat less polymer is required and the method of our invention is notdependent upon the nature of the rock surface for absorption, and sweepefficiencies are not limited by the pseudoplastic nature of the viscouspolymer solutions.

The method of our invention injects a solution of a mixture of solublebivalent metal salt capable of being oxidized and high molecular weightpolymer having groups chemically reactive with trivalent metal ions toform a plugging agent. The method of our invention provides saidplugging agent for treating high permeability strata through whichwaterflooding tends to bypass crude oil in waterflooding operations.

The method of our invention is carried out in the absence of air untilthe calculated time of effective plugging is reached, then air isinjected into the same high permeability strata with the air fingeringthrough the said strata and the contained polymer-bivalent metal ionsolution causing oxidation thereof to the trivalent form. The oxidationresults in precipitation of a gelatinous material thereby reducingfurther flow of water and forcing the water to displace oil from lesspermeable zones of the porous reservoirs.

An object of this invention is to provide a method for controlling thepermeability of a subterranean formation in waterflooding operations.Another object of this invention is to provide a method for selectivelyplugging the more permeable water channels of a subterranean formation.

In particular, the method of our invention comprises injecting into aninput well penetrating the subterranean, compact, oil'bearing formation,also penetrated by output wells, an aqueous flooding media beingsubstantially free of molecular oxygen and having dissolved therein awater-soluble polymer and a water-soluble bivalent metal salt. Thewater-soluble polymer consists of a high molecular weight polymer havinggroups chemically reactive with trivalent metal ions whereupon such areaction causes gelation or precipitation of an insoluble gel. Theaforementioned polymer does not precipitate and is not reactive withbivalent metal ions present in the solution as injected into the well.Air is excluded in the first step of the method of our invention fromthe solution to avoid or minimize oxidation of the bivalent metal ions.Subsequently, air is injected into the same high permeability-strata asthe aforementioned waterflood solutions, with the air fingering throughthe polymer-bivalent metal ion solution therein causing oxidation of thebivalent metal ion to the trivalent form, resulting in precipitation ofa gelatinous material which reduces further flow of water through saidstrata. The reduction of the waterflow through the high permeabilitystrata forces the water to flow through other strata therein displacingoil from less permeable zones of the porous reservoir. Practice of theforegoing method will produce excellent recovery of oil with theemployment of a minimum amount of flooding medium additive.

The condition requiring the aqueous flooding media to be substantiallyfree of molecular oxygen is achieved by several methods. A direct andconventional technique is to utilize well water for making up theflooding media. By well water is meant any water produced from asubterranean formation. While in the earth, water is normally underreducing conditions and thus, if when brought to the surface, it ismaintained in a closed system whereby oxygen is excluded, the oxygenlevel will be maintained at a satisfactory low level. By whatevertechnique the waterflood base is deoxygenated it is desirable ashereinbefore described, to maintain the oxygen content from about 0.1 to4.0 parts per million depending upon the trivalent salt used.

Polymers that can be employed in the invention are watersoluble organicpolymers, sometimes referred to as hydrophilic polymeric colloids,characterized by a substantial linearity, and a high molecular weight.The term water soluble as employed in the foregoing technology meansdispersable in water to provide a visually homogeneous system definitelydiluted with water. In the method of our invention a given minimummolecular weight would be arbitrary, especially when used to define thetransition point between low polymers and macromolecules. The polymersused in our invention are to be considered as having high molecularweights averaging above about 100,000. Suitable polymers as describedhereinabove which may be utilized in the method of our invention are forexample, carboxylated cellulose, partly hydrolyzed polyacrylamide, andthe like.

The method of our invention provides a flexible method for forming gels,in situ, under ambient conditions. Suitable trivalent metal salts foruse in the method of this invention include the trivalent metal salts ofacids, e.g., ferric sulfate, ferric chloride, ferric nitrate, chromicchloride, chromic acetate, and the like which are produced from theirrespective bivalent forms upon contact with oxygen. The amount of metalsalt to be used will be a sufficient amount, e.g., for effecting thegelatinous precipitation as described hereinabove. For example, theratio of a ferrous salt to polymer by weight is from about 1.0 to about0.006. Circumstances will dictate changes from the above ferrous salt topolymer e.g., when other of the aforementioned salts and mixturesthereof are used as desired.

The method of our invention overcomes the limitation of injecting two ormore slugs of different liquids into the formation and expectingfingering or mixing in the formation which may not occur. Air is easilyinjected and readily mixes and fingers into the formation and formationliquids to convert the bivalent metal ions to trivalent form. Thebivalent metal ions have been previously mixed into the polymer at thesurface and when converted to trivalent ions reaction occurs in thepolymer to produce gels. Clearly, this invention can be extended tocover the use of other bivalent metal salts which can readily beoxidized to trivalent ions for reaction with any polymer to produce gelsin waterflooding. The distance at which gels are produced from the wellbore can thus be readily controlled by merely determining when to injectair. The polymer-bivalent metal salt solution additive may be, injectedinto the well bore in slugs or in a continuous fashion along with thewater of the waterflood.

From the foregoing description, it should be apparent that a newtechnique for plugging high permeability strata and waterfloodingoperation has been developed which offers significant advantages overprior art techniques.

What we claim is:

1. An in situ gelation method for waterflood plugging, comprising:

introducing a solution containing a water-soluble bivalent metal saltand a water-soluble, substantially linear polymer of high molecularweight having groups chemically reactive with trivalent metal ions to awaterflood zone in the absence of air;

injecting air into the waterflood zone containing the bivalent metalsalt and the polymer solution;

oxidizing the bivalent metal content of said zone to the trivalent form;

precipitating an insoluble gelatinous form of the reacted trivalentmetal ion-polymer contents of said waterflood I polyacrylamide.

3. A method-according to claim 1 wherein the bivalent metal salt isselected from at least one of ferrous sulfate, ferrouschloride, ferrousnitrate, chromous chloride, chromous nitrate, and chromous acetate.

4. A method according to claim 1 wherein the bivalent metal salt is aferrous salt and the ratio of ferrous salt to polymer by weight is fromabout 1.0 to about 0.006.

5. A method according to claim 1 wherein the oxygen content of thesolution containing the water-soluble bivalent metal salt and thewater-soluble polymer is from about 0.1 to about 4.0 parts per million.

6. A method according to claim 1 wherein the water-soluble bivalentmetal salt and water-soluble polymer solution is introduced to thewaterflood zone in a continuous method.

7. A method according to claim. 1 wherein the water-soluble bivalentmetal salt and water-soluble polymer solution is introduced to thewaterflood zone through a slug method.

2. The method according to claim 1 wherein the polymer has a molecularweight of at least 100,000 and is selected from at least one ofcarboxylated cellulose and partly hydrolyzed polyacrylamide.
 3. A methodaccording to claim 1 wherein the bivalent metal salt is selected from atleast one of ferrous sulfate, ferrous chloride, ferrous nitrate,chromous chloride, chromous nitrate, and chromous acetate.
 4. A methodaccording to claim 1 wherein the bivalent metal salt is a ferrous saltand the ratio of ferrous salt to polymer by weight is from about 1.0 toabout 0.006.
 5. A method according to claim 1 wherein the oxygen contentof the solution containing the water-soluble bivalent metal salt and thewater-soluble polymer is from about 0.1 to about 4.0 parts per million.6. A method according to claim 1 wherein the water-sOluble bivalentmetal salt and water-soluble polymer solution is introduced to thewaterflood zone in a continuous method.
 7. A method according to claim 1wherein the water-soluble bivalent metal salt and water-soluble polymersolution is introduced to the waterflood zone through a slug method.